1. Field of the Invention
The present invention relates to a developing device for use in an image forming apparatus and to an image forming apparatus including the developing device.
2. Description of the Related Art
An electrophotographic or electrostatic recording image forming apparatus, such as a copier, a printer, or other apparatuses electrostatically charges and then exposes an electrophotographic photosensitive member (photosensitive member) serving as an image carrier, thereby forming an electrostatic image (latent image) thereon. By developing the latent image by the use of toner being a developer, the apparatus forms toner images on the image carrier. The toner image is transferred to a transfer material (recording medium), such as a recording paper, a transparency, or other materials, directly or after being temporarily transferred to an intermediate transfer material. By fixing the toner image by the use of heat, pressure, and/or other elements, the apparatus can generate a recording image.
The photosensitive member and at least one of a charging unit, a developing unit, and a cleaning unit, which serve as a process unit acting on the photosensitive member, can be integrally combined into a cartridge so as to constitute a process cartridge attachable to and removable from a main body of the image forming apparatus. Alternatively, the developing unit can be singularly formed as a cartridge attachable to and removable from the main body. Such a cartridge system further enhances operability, so a user can easily perform maintenance of the process unit. Therefore, the cartridge system is widely used in electrophotographic image forming apparatuses.
One example of systems for developing a latent image on an image carrier is a system that performs development by applying an alternating electric field (oscillating electric field) between the image carrier and a developer carrier included in the developing device, the image carrier and the developer carrier being not in contact with each other. For this system, development is performed by attaching a developer flying/dispersed from the developer carrier to an electrostatic latent image formed on the image carrier.
In the case where the developer is dispersed from the developer carrier to the image carrier, even when the latent image to have consistent density has been formed on the image carrier to which the developer is to move, only the trailing end of an image can have a higher density because a larger amount of the developer is attached to the trailing end of the latent image. The image defects are called “sweep-together phenomenon.” The sweep-together phenomenon is described below with reference to FIG. 16. FIG. 16 is a model diagram of a cylindrical photosensitive member (photosensitive drum) 101 serving as the image carrier and a development roller 2 serving as the developer carrier viewed from the longitudinal direction.
Sweep-together phenomena are phenomena in which a large amount of toner is gathered at the trailing end of an image in the direction of movement of the surface of the photosensitive drum 101, as indicated with a reference numeral H in FIG. 16. The toner image like this causes an image defect of inconsistencies in density in that one area is denser than the other areas.
As illustrated in FIG. 16, when an AC bias is applied to the development roller 2, a barrel-shaped electric field D occurs between the photosensitive drum 101 and the development roller 2. Toner T attached to the surface of the development roller 2 reciprocates between the photosensitive drum 101 and the development roller 2 along electric lines of force formed by an electric field. If the barrel-shaped electric field D described above is formed, the toner T moves outwardly from a closest point S of contact between the photosensitive drum 101 and the development roller 2. In other wards, when an AC bias is applied to the development roller 2, the toner T1 within a developing area G has a velocity component that always moves in a direction outward from the developing area G.
A case is described below in which the photosensitive drum 101 and the development roller 2 are rotating in the direction of the arrows in FIG. 16, and a latent image is being formed on the photosensitive drum 101, that is, an operation during an actual development is described. In FIG. 16, a region with a potential of −100 V on the photosensitive drum 101 is a latent-image part and an area on which a toner image is to be formed. A region with a potential of −500 V on the photosensitive drum 101 is a reference-potential part of the photosensitive drum 101 and an area on which the toner image is not to be formed.
When the latent-image part reaches the inside of the developing area G, the toner T on the development roller 2 is attaching to the latent-image part. However, since the flying toner T1 has a velocity component that moves in the direction outward from the developing area G, as described above, the toner T1 moves upstream of the latent-image part. At the border between the region having a potential of −100 V and the region having a potential of −500 V on the photosensitive drum 101, an electric field from the region having a potential of −500 V to the region having a potential of −100 V occurs. As a result, the toner T1 that has moved upstream of the latent-image part stops at this border. The amount of toner at the trailing end of the latent-image part is larger than that at the leading end and the central portion, so that the region H where toner is swept together (swept-together image H) is formed.
A method for reducing a sweep-together phenomenon by the provision of a plate member between an image carrier and a developer carrier is disclosed in Japanese Patent Laid-Open No. 8-022185.
The provision of the plate member between the image carrier and the developer carrier for reducing a sweep-together phenomenon requires the leading end of the plate member to be located within an AC electric field. This may cause the plate member to resonate with the AC electric field and thus increase the vibration of the plate member, so that an unpleasant high-frequency noise (hereinafter referred to as “AC noise” or “development noise”) may be produced.
One approach to this problem is to slightly increase the thickness of the plate member or make a slit in the base thereof to avoid resonance and to lower AC noise relating to an AC bias having a predetermined frequency.
However, the coincidence between the natural frequency of the plate member and the frequency of the AC bias may cause resonance of the plate member and produce noise. In addition, in an image forming apparatus, the frequency of the AC bias applied to the developer carrier may be changed. For example, according to the temperature and/or humidity of the inside of the image forming apparatus, the frequency of the AC bias applied to the developer carrier may be changed. In this case, the plate member is more apt to resonate, compared with a case in which the frequency is not changed.
There is no consideration on efficient reduction in the AC noise in the case where the frequency of the AC bias applied to the developer carrier is changed.